1. Field of the Invention
The present invention relates to a process of producing a fluorophosphate glass and more particularly to a process of producing a fluorophosphate glass having a high positive anomalous partial dispersion + .DELTA..nu..sub.e and relatively high indices of refraction n.sub.e with Abbe numbers .nu..sub.e between 84.1 and 51.0 and to such an optical fluorophosphate glass.
2. Description of the Prior Art
Heretofore crystals -- such as, for instance, crystals of calcium fluoride, i.e. of fluorite (CaF.sub.2) -- namely crystals having an extremely highly positive anomalous partial dispersion value, were required in micro-optical as well as macro-optical apochromatic systems in which the so-called "secondary spectrum" was corrected. The three optical parameters for the mineral fluorite which are of interest in this connection are as follows:
______________________________________ Index of refraction n.sub.e = 1.4349 Abbe-value .nu..sub.e = 94.8 Partial dispersion .DELTA..nu..sub.e = +31.5 ______________________________________
However, the use of said natural or synthetic mineral in optical systems is accompanied by considerable difficulties in the manufacturing process because the properties of said material are rather unsatisfactory. For instance, cubic fluorite has a pronounced ability of being split along its (111)-direction. Furthermore, it has a low hardness, namely a hardness of 4 according to the Mohs' hardness scale. Said low hardness results in an early destruction of its surface areas. Finally its enormously high price prohibits its use in many modern optical systems.
Therefore, attempts have been made to produce, by melting, optical glasses with similar optical parameters and to utilize such glasses technologically in the manufacture of highly valuable, completely corrected objectives.
For producing such known glasses there are used as glass-forming agents preferably phosphates of the I., II., and/or III. Group of the Periodic System of chemical elements. For instance, German Pat. No. 1,596,877 describes a fluorophosphate glass which contains 9 mol % to 12 mol % of metaphosphates of the monovalent elements lithium and/or sodium and/or potassium as well as of the bivalent alkaline earth metal element magnesium and of the trivalent element aluminum. In such glasses the proportion of aluminum metaphosphate must at least amount to half of the entire phosphate amount.
Furthermore, glasses in which beryllium fluoride (BeF.sub.2) is used as glass-forming component, are known. These glasses, however, have not achieved any technological importance due to their very high toxicity and the inherent perils encountered during the melting process when using such glass-forming components. Furthermore, these beryllium fluoride-containing phosphate glasses have the tendency to devitrify very considerably, especially when larger melt charges are molten.
The known glasses have the great disadvantage that they show relatively numerous inhomogeneities which are due to the technology employed during melting on account of the inherent chemical composition of the melt.